Heavy austempered ductile iron components

ABSTRACT

A component for wind turbines includes cast austempered ductile iron containing about 3.0 to about 3.8 weight percent carbon, about 1.9 to about 2.8 weight percent silicon, up to about 0.3 weight percent manganese, up to about 0.8 weight percent copper, up to about 2.0 weight percent nickel, up to about 0.3 weight percent molybdenum, about 0.03 to about 0.06 weight percent magnesium, less than about 0.05 weight percent chromium, less than about 0.02 weight percent vanadium, and less than about 0.01 weight percent sulfur. The component is preferably a drive shaft or gearbox component having a mass of more than about 3 tons. A method of manufacturing the component is also provided.

BACKGROUND OF THE INVENTION

The invention relates to austempered ductile iron for use in wind turbine shafts and gearbox components.

Wind turbines have a main shaft that transmits power from a rotor to a generator. As wind turbines increase their outputs from 1.5 and 2.5 megawatts (MW) to 3, 4, 5, and 6 MW, the size and required properties of the wind turbine drive shaft increases. In addition, the loads gearbox components, such as planet gear carriers, handle are too high for conventional ductile iron grades (ferrtitic/pearlitic grades). Forged/hardened steel is the material of choice for gearbox components and drive shafts having sizes greater than 3 tons. This shaft is typically machined out of a steel forging. The material of the shaft is usually quenched-tempered high-strength low alloy steel with critical fatigue properties. Examples include nickel chromium steels such as 34CrNiMo6 steel.

Processing of heavy forged steel wind components from large ingots is complex, requiring numerous hot working operations (a.k.a., forging) and heat treatment operations to sufficiently refine the structure to provide a suitable microstructure responsive to the subsequent quality heat treatment to develop desired mechanical properties. Such exhaustive processing paths and the extensive machining attributed to the limited freedom of geometry come at a steep cost relative to the straightforward production of cast ADI counterparts.

SUMMARY OF THE INVENTION

Embodiments of the invention include a component that is cast austempered ductile iron containing about 3.0 to about 3.8 weight percent carbon, about 1.9 to about 2.8 weight percent silicon, up to about 0.3 weight percent manganese, up to about 0.8 weight percent copper, up to about 2.0 weight percent nickel, up to about 0.3 weight percent molybdenum, about 0.03 to about 0.06 weight percent magnesium, less than about 0.05 weight percent chromium, less than about 0.02 weight percent vanadium, and less than about 0.01 weight percent sulfur. The component has a mass of more than about 3 tons.

Another embodiment of the invention includes a method of manufacturing a component. The method includes melting ductile iron containing about 3.0 to about 3.8 weight percent carbon, about 1.9 to about 2.8 weight percent silicon, up to about 0.3 weight percent manganese, up to about 0.8 weight percent copper, up to about 2.0 weight percent nickel, up to about 0.3 weight percent molybdenum, about 0.03 to about 0.06 weight percent magnesium, less than about 0.05 weight percent chromium, less than about 0.02 weight percent vanadium, and less than about 0.01 weight percent sulfur. The component is cast. The component is austenitized and quenched to the austempering temperature. The component is austempered.

Embodiments of the invention include a wind turbine drive shaft that is cast austempered ductile iron containing about 3.0 to about 3.8 weight percent carbon, about 1.9 to about 2.8 weight percent silicon, up to about 0.3 weight percent manganese, up to about 0.8 weight percent copper, up to about 2.0 weight percent nickel, up to about 0.3 weight percent molybdenum, about 0.03 to about 0.06 weight percent magnesium, less than about 0.05 weight percent chromium, less than about 0.02 weight percent vanadium, and less than about 0.01 weight percent sulfur. The drive shaft has a mass of more than about 3 tons.

DETAILED DESCRIPTION

Austempered ductile iron (ADI) that is cast is able to provide high mass and net-shaped components, greater than about 3 tons, more preferably greater than about 6 tons, for heavy wind turbine shafts and gearbox components. Presently, forge/hardened steel is used to make large wind turbine shafts and gearbox components. However, forged/hardened steel, for example 34CrNiMo6, is a relatively expensive material that requires a complex process to produce a component, especially a component of greater than about 3 tons with complex geometries. As size increases to 6 tons or greater the expense of producing a forged/hardened steel component is even greater and the worldwide supplier base is very limited. The typical steps required to produce heavy wind turbine gearbox components from high strength, low alloy steels include melting of ingot, cogging of ingot into billet, forging of billet, forging of part, normalizing, austenitizing, water quenching, tempering and extensive/complicated machining. The process to produce gearbox components from high strength, low-alloy steel requires numerous steps and a large energy requirement when compared with a conventional casting process. These manufacturing steps for producing components from high strength, low-alloy steel increase in cost as the size of the component increases.

The primary chemical composition of austempered ductile iron (ADI) used in embodiments of the invention includes about 3.0 to about 3.8 weight percent (w/o) carbon, about 1.9 to about 2.8 weight percent silicon, up to about 0.3 weight percent manganese, up to about 0.8 weight percent copper, up to about 2.0 weight percent nickel, up to about 0.3 w/o molybdenum, less than about 0.05 weight percent chromium, less than about 0.02 weight percent vanadium, about 0.03 to about 0.06 weight percent magnesium and less than about 0.01 weight percent sulfur. Primary chemistry is used to identify the most important elements. Not every element is identified as there are certain “tramp” elements at low concentrations in the iron. The term ductile iron means that iron makes up the remainder of the composition except for “tramp” elements. ADI provides weight reduction attributed to its lower density, noise reduction attributed to its higher damping capability, similar or better mechanical properties than those of cast/forged steel and the casting processes provide less costly manufacturing and keeps machining of the component to a minimum. ADI requires a special isothermal heat treatment, referred to as austempering, which provides excellent combinations of high strength and toughness. The strain induced by the final machining after this heat treatment enhances the fatigue properties.

Certain properties of ADI are not as good as forged/hardened steel. ADI possesses 15-20 percent less stiffness than forged/hardened steel and lower impact resistance. However, by making the parts slightly larger and/or proper design modifications this deficiency is mitigated. In comparing ADI with forged/hardened steel and the chemistry range identified, it is possible to obtain heavy wind turbine shafts and gearbox components at reduced costs with comparable properties.

The preferred primary chemical composition of ADI for use in embodiments of the present invention includes about 3.0 to about 3.8 weight percent (w/o) carbon, about 1.9 to about 2.8 weight percent silicon, up to about 0.3 weight percent manganese, up to about 0.8 weight percent copper, up to about 2 weight percent nickel, up to about 0.3 weight percent molybdenum, about 0.03 to about 0.06 weight percent magnesium, less than about 0.05 weight percent chromium, less than about 0.02 weight percent vanadium, and less than about 0.01 weight percent sulfur. As mentioned above, iron is the remaining constituent except for certain tramp elements.

Carbide forming elements including but not limited to chromium and vanadium shall be held to as low a level as possible to avoid formation of massive carbides in the microstructure. Chromium and vanadium should be below 0.05 weight percent and 0.02 weight percent respectively.

The matrix microstructure of ADI includes a fine-scale dispersion of acicular ferrite with ductile high carbon stabilized austenite, so called ausferrite. The ausferrite matrix is responsible for unique properties attainable in ADI components.

The austenite in ADI is thermally stabilized with carbon during heat treatment and will not transform to brittle martensite even at temperatures approaching absolute zero. Stable, carbon enriched austenite can undergo a strain-induced transformation when exposed to high, normal forces. This transformation, which gives ADI its remarkable wear resistance, is more than mere “work hardening”. In addition to a significant increase in flow stress and hardness, this strain induced transformation of the austenite to martensite also produces a localized increase in volume and creates high compressive stresses in the “transformed” areas. These compressive stresses inhibit crack formation and growth, and produce significant improvements in the fatigue properties of ADI when it is machined after heat treatment or subjected to surface treatments such as shot peening, grinding or rolling.

Austenitizing is the process of holding the ductile iron casting above the critical temperature for a sufficient period of time to ensure that the matrix is fully transformed to austenite. Both austenitizing time and temperature depend on the microstructure and composition of the as-cast material. In order to produce a single phase matrix microstructure (austenite) with a uniform carbon distribution, austenitizing includes holding the casting at temperatures in the range of about 815-985° C. (1500-1800° F.) for a time period that is sufficient to fully convert the matrix of the thickest section to austenite. Unlike steels, selection for the austenitizing temperature in cast irons determines the initial carbon content of the austenite, a factor crucial in defining the thermodynamic driving force for ausferrite transformation during subsequent austempering. Additionally, proper selection of the austenitizing temperature will help ensure distribution of austenite and ferrite phases within the fully transformed ausferrite product.

An isothermal hold is preformed right after a direct quench from austenitizing to a temperature above the martensitic transformation. The rate of quench has to be high enough to avoid ferrite/pearlite formation. Depending upon the desired mechanical properties of end products, the temperature of the isothermal hold (a.k.a. austempering) is in the range of about 230 to about 400° C. (450-750° F.) and its duration has to be long enough to produce a matrix of ausferrite which includes acicular ferrite and austenite stabilized with about 2 weight percent carbon. Selection of the austempering temperature also plays a crucial role in defining the nature and composition of the ausferrite product and its attendant properties. In general, as the austempering temperature decreases, the carbon stabilized austenite will increase in carbon content and decrease in volume fraction, which is compensated by an increase in volume fraction of ferrite. Ductile iron austempered at lower temperatures will typically have a finer matrix microstructure with improved strength compared to iron transformed at higher temperatures. However, these irons will typically contain higher levels of carbide at the expense of ferrite potentially leading to a drop in toughness. Careful selection of austempering temperature with sufficient hold time is required to develop the desired properties of ADI components.

Generally, cast ADI parts provide components that are net-shaped and require no further machining; however, for some parts, depending on their geometry machining may be necessary. Gun drilling or deep hole drilling that is needed to create a center bore in forged wind turbine shafts can be eliminated with the use of cores in the casting process described above.

The terms “first,” “second,” and the like, herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another, and the terms “a” and “an” herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item. The modifier “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context, (e.g., includes the degree of error associated with measurement of the particular quantity). The suffix “(s)” as used herein is intended to include both the singular and the plural of the term that it modifies, thereby including one or more of that term (e.g., the metal(s) includes one or more metals). Ranges disclosed herein are inclusive and independently combinable (e.g., ranges of “up to about 25 w/o, or, more specifically, about 5 w/o to about 20 w/o”, is inclusive of the endpoints and all intermediate values of the ranges of “about 5 w/o to about 25 w/o,” etc).

While various embodiments are described herein, it will be appreciated from the specification that various combinations of elements, variations or improvements therein may be made by those skilled in the art, and are within the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. 

1. A component comprising: cast austempered ductile iron containing about 3.0 to about 3.8 weight percent carbon, about 1.9 to about 2.8 weight percent silicon, up to about 0.3 weight percent manganese, up to about 0.8 weight percent copper, up to about 2.0 weight percent nickel, up to about 0.3 weight percent molybdenum, about 0.03 to about 0.05 weight percent magnesium, less than about 0.05 weight percent chromium, less than about 0.02 weight percent vanadium, and less than about 0.01 weight percent sulfur wherein the component comprises a mass of more than about 3 tons.
 2. The component of claim 1, wherein the component comprises a mass of more than about 6 tons.
 3. The component of claim 1, wherein the component comprises a drive shaft.
 4. The component of claim 1, wherein the component comprises a heavy gearbox component.
 5. The component of claim 1, wherein the austempered ductile iron comprises acicular ferrite and austenite.
 6. A method of manufacturing a component, the method comprising: melting ductile iron containing about 3.0 to about 3.8 weight percent carbon, about 1.9 to about 2.8 weight percent silicon, up to about 0.3 weight percent manganese, up to about 0.8 weight percent copper, up to about 2.0 weight percent nickel, up to about 0.3 weight percent molybdenum, about 0.03 to about 0.06 weight percent magnesium, less than about 0.05 weight percent chromium, less than about 0.02 weight percent vanadium, and less than about 0.01 weight percent sulfur; casting the component; austenitizing the component; quenching the component; and austempering the component.
 7. The method of claim 6, wherein the quenching comprises immersing the component in a medium kept at a temperature of from about 230 to about 400° C.
 8. The method of claim 6, further comprising: machining the component.
 9. The method of claim 6, wherein the component comprises a mass of more than about 3 tons.
 10. The method of claim 6, wherein the component comprises a mass of more than about 6 tons.
 11. The method of claim 6, wherein the component comprises a wind turbine shaft.
 12. The method of claim 6, wherein the component comprises a gearbox component.
 13. The method of claim 6, wherein the austenitizing comprises holding the component at a temperature in a range of about 815° C. to about 985° C. for a time sufficient to convert a matrix to austenite
 14. A wind turbine drive shaft comprising: cast austempered ductile iron containing about 3.0 to about 3.8 weight percent carbon, about 1.9 to about 2.8 weight percent silicon, up to about 0.3 weight percent manganese, up to about 0.8 weight percent copper, up to about 2.0 weight percent nickel, up to about 0.3 weight percent molybdenum, about 0.03 to about 0.06 weight percent magnesium, less than about 0.05 weight percent chromium, less than about 0.02 weight percent vanadium, and less than about 0.01 weight percent sulfur wherein the drive shaft comprises a mass of more than about 3 tons.
 15. The wind turbine drive shaft of claim 14, wherein the drive shaft comprises a mass of more than about 6 tons.
 16. The wind turbine drive shaft of claim 14, wherein the austempered ductile iron comprises acicular ferrite and austenite. 